1. Field of the Invention
The present invention relates to luminescent polymers suitable for use in organic light-emitting devices (OLEDs), organic solid-state lasers, photovoltaic cells, and electrochromic displays. The invention further relates to a method of producing the luminescent polymers, and of producing OLEDs, organic solid state lasers, photovoltaic cells and electrochromic displays employing such luminescent polymers in an active layer.
2. Description of the Related Art
Conjugated polymer semi-conductors have attracted attention for optoelectronic applications that traditionally have been the domains of inorganic semi-conductors. The structural flexibility, the very low cost for processing, and the flexible thin film features of conjugated polymers make organic semi-conductive polymer devices competitive with the inorganic semiconductors. Most conjugated polymers are highly luminescent, and they have attracted great attention in light-emitting device applications and for use in organic solid-state lasers.
Recent improvements in performance characteristics of organic/polymer LEDs have led to renewed interest in organic lasers. Laser emission in general relies on two key ingredients: first, the active material must exhibit strong stimulated emission. Second, there has to be an optical feedback mechanism. The first requirement is fulfilled for a variety of state-of the-art conjugated polymers. The observation of gain narrowing via amplification of spontaneous emission (ASE) in planar conjugated polymer wave guides has proven that conjugated polymers can exhibit high optical gain comparable to the best inorganic semiconductor laser materials.(M. D. McGehee and A. Heeger, Adv. Mater., 2000, 12(22), 1655-1668). The second important condition for lasing, namely an optical feedback, has to be accomplished by a suitable resonator geometry which is dependent on physical device configuration. Recent realization of the first organic solid state injection laser pumped by electrical charge injection reveals that the current is at the level of more than 1000 A/cm2 for an electrically pumped organic solid state laser.(J. H. Schon, et al, Science, 2000, 278, 599-601). This is considerably much more than a normal OLED which is operated at a level of 10-100 mA/cm2 (for a brightness between 100-1000 cd/m2). The prospect of compact electrically pumped polymer lasers using their semi-conductor properties is exciting. However, there is indeed a need to develop highly luminescent and highly stabilized polymers.
Luminescent conjugated polymers are generally known in the art and include substituted and unsubstituted poly(phenylene vinylene)s and polyphenylenes. They are widely used in active layers which can be incorporated into a variety of OLED, organic solid-state laser, photovoltaic cell and electrochromic display structures. For all the above applications, polymers are either subject to electrical field, charge injection, or light irradiation conditions. Besides having the luminescence property, the other key parameter for these applications is the stability of the material under conditions of use, including high UV irradiation and high currents. Thus, the materials must maintain a high level of photoluminescent properties in use, as well as maintain their structural integrity.
Efforts to use these conjugated polymers commercially have focused on improving stability. It has been found that stability can be improved by the introduction of highly hindered aromatic rings, such as diphenyl-substituted poly(phenylene vinylene)s and phenyl-substituted poly(phenylene vinylene)s.(U.S. Pat. No. 5,945,502). Even with these improvements, however, stability is still unsatisfactory for use under certain conditions including high UV irradiation and high currents, both of which are normal conditions in OLEDs, organic solid state lasers, photovoltaic cells, and electrochromic displays.